In high performance hot blast stoves which are operated at very high temperatures, damage occurs due to intergranular stress corrosion cracking. The damage is caused by the increased formation of nitrogen oxides in the hot blast, heated to temperatures of over 1 300.degree. C., and by the occurrence of a nitrate-containing electrolyte when the blast moisture condenses on those structural plant components of the hot blast stoves which are conventionally made from unalloyed or low-alloy steels.
A measure for guarding against stress corrosion cracking which has already been successfully used for two decades s0 is to apply an outer thermal insulation (external insulation) by means of which the sheet metal temperature can be raised high enough to prevent the separation of the condensate which causes stress corrosion cracking.
High-alloy steels, such as stainless CrNiMo steels have also been successfully used., for example, for the particularly endangered heavily stressed compensators in the pipe systems of hot blast stoves or as a cladding material for sheet metal.
However, it is very expensive to equip a hot blast stoves with external insulation and to use stainless steels, so that an attempt is still being made to discover a steel alloy which is sufficiently resistant to stress corrosion cracking at a justifiable alloying expense.
German Patent 29 07 152 discloses a steel for the lining of furnaces, boilers and high temperature heaters in which nitrogen oxygen-containing combustion gases occur. The steel contains additions of chromium, molybdenum and niobium: (carbon +nitrogen) ratio must not be higher than 7. While the alloying elements chromium and molybdenum are important for the formation of a passive layer on the surface of the steel, niobium is intended to fix a proportion of the carbon and nitrogen, to prevent chromium impoverishment at the grain boundaries during welding or heating. The sum of carbon and nitrogen shall not be above 0.06%. With regard to the stoichiometric ratio, there is a deficiency of niobium as against carbon and nitrogen, so that inevitably chromium carbides and carbonitrides must also be formed. Titanium is mentioned as a further carbide and nitride-forming element., but it is not supposed to be as effective as niobium.
German Patent 28 19 227 discloses a manganese steel to be used in the normalized condition as a material for structural members which are exposed to alkaline, neutral or weakly acid solutions, more particularly for hot blast stoves. The steel contains a relatively high carbon content up to 0.18% and adjusted contents of phosphorus and sulphur in addition to manganese, niobium and copper, to prevent intergranular hydrogen-induced cracks. The steel can also optionally contain nickel, chromium and titanium. For the welding of the steel a complicated method is disclosed for achieving a higher resistance in welded constructions to stress corrosion cracking and other crack formation.
Resistance in nitrate or alkali media is defined in German Standard DIN 50915, but this Standard no longer corresponds to the present state of art. It has been found that steels shown to be resistant by this standard test were in fact not resistant under severe attack. Severe corrosion testing is performed in synthetic hot blast condensates or corresponding nitrate solutions at a constant critical strain rate of 10.sup.-6 to 10.sup.-7 /sec. It therefore might be that a steel as disclosed in German Patent 28 19 227 does not fit when expoxed to practical conditions in a hot blast stove.
In the German Journal "Werkstoffe und Korrosion" (="Materials and Corrosion"), 20 (1969), No.4, pages 305 to 313 under the title "The present state of knowledge concerning the stress corrosion cracking of unalloyed and low-alloy steels" is disclosed that an increasing carbon content has very favourable effect on resistance to stress corrosion cracking, while steels having carbon contents of around or below 0.2% are supposed to be particularly sensitive. An improving effect is ascribed to titanium, in addition to other elements. The material given as an example, a soft iron containing 0.46% titanium, however, is so remote from actual steels and so problematic due to the very high titanium content as regards production, properties and costs, that it cannot be regarded as a starting point for a technical solution to the problem.
The statement that the stable fixation of carbon and nitrogen enhances resistance to stress corrosion cracking relates to the attack of solutions of alkalies, while nitrate solutions occur in hot blast stoves.
The Journal "Corrosion", (1981)., pages 650 to 664 publishes an assessment of the Literature and a comprehensive systematic investigation of the effect of chemical composition on the stress corrosion cracking of unalloyed and low-alloyed steels. One general conclusion drawn by the Paper is that chromium and titanium enhance resistance to stress corrosion cracking, the result concerning the effect of titanium deserving special attention, since the Literature and the experimental results which the Paper presents lead to the conclusion that any significant effect on resistance to stress corrosion cracking can be detected only with very high alloying contents of about 1% titanium. It is true that as regards the effect of the carbon content the Publication draws attention to the favourable corrosion behaviour of soft iron having a very low carbon content and 0.46 % titanium, but the basic message of this Publication, which agrees with other Literature, is the favourable effect of increasing carbon content on resistance to stress corrosion cracking. This is clearly expressed in the regression equation, in which resistance to stress corrosion cracking in nitrate solutions is demonstrated with increasing contents not only of titanium and chromium, but also of carbon. A similar effect is also ascribed to the nitrogen content. However, the emerging idea of making a steel more resistant to stress corrosion cracking by as high contents of titanium, carbon and nitrogen as possible comes up against considerable practical and economic problems.
The manufacturing difficulties and very heavy costs of such a steel are unjustifiable Surprisingly, the invention now shows that very satisfactory resistance to stress corrosion cracking can be achieved by limiting the carbon and nitrogen content to the lowest possible level and adapting thereto a titanium content of the order of magnitude of 0.1 to 0.2 %.
It is an object of the invention to provide a structural steel which can be welded by a very simple process and which has high resistance to stress corrosion cracking, more particularly in nitrate solutions, at a low expense of alloying elements and also has satisfactory toughness and ductility.